R Language Basic use and chaining


The pipe operator, %>%, is used to insert an argument into a function. It is not a base feature of the language and can only be used after attaching a package that provides it, such as magrittr. The pipe operator takes the left-hand side (LHS) of the pipe and uses it as the first argument of the function on the right-hand side (RHS) of the pipe. For example:


1:10 %>% mean
# [1] 5.5

# is equivalent to
# [1] 5.5

The pipe can be used to replace a sequence of function calls. Multiple pipes allow us to read and write the sequence from left to right, rather than from inside to out. For example, suppose we have years defined as a factor but want to convert it to a numeric. To prevent possible information loss, we first convert to character and then to numeric:

years <- factor(2008:2012)

# nesting

# piping
years %>% as.character %>% as.numeric

If we don't want the LHS (Left Hand Side) used as the first argument on the RHS (Right Hand Side), there are workarounds, such as naming the arguments or using . to indicate where the piped input goes.

# example with grepl
# its syntax:
# grepl(pattern, x, ignore.case = FALSE, perl = FALSE, fixed = FALSE, useBytes = FALSE)

# note that the `substring` result is the *2nd* argument of grepl
grepl("Wo", substring("Hello World", 7, 11))

# piping while naming other arguments
"Hello World" %>% substring(7, 11) %>% grepl(pattern = "Wo")

# piping with .
"Hello World" %>% substring(7, 11) %>% grepl("Wo", .)

# piping with . and curly braces
"Hello World" %>% substring(7, 11) %>% { c(paste('Hi', .)) }
#[1] "Hi World"

#using LHS multiple times in argument with curly braces and .
"Hello World" %>% substring(7, 11) %>% { c(paste(. ,'Hi', .)) }
#[1] "World Hi World"